Chapter 14: Logging and Light Observability
14.11c From red metric to log line (glue with Chapter 13)
Practise this joint drill once:
- Notice a saturation or error signal on your Grafana (or CloudWatch) view.
- Note the IST time window.
- Jump to logs for the same window (
docker logs, journal, or CloudWatch filter). - Find one ERROR/WARN that explains the graph.
- Write the five-line incident note.
That glue is "light observability" for juniors: not a full APM suite — just the habit of connecting graph → evidence → action.
| Detect (ch13) | Explain (ch14) | Act |
|---|---|---|
| Disk % climbing | journal / app logs show rotate failure |
Fix rotate; free space; confirm alert clears |
| Error rate spike | CloudWatch shows db_timeout |
Rollback SHA; ticket for DB pool |
| Latency high | Logs show slow /badge queries |
Index / cache follow-up — after lunch stabilises |